Dual phase steels have not shown superior characteristics with regard to the properties such as hole expansion, Charpy impact, fatigue and especially flash butt welding which is closely related to wheel rim fabrication. Through the investigation of the nature and volume fraction of second phase and alloying elements, it has been clarified that Nb bearing steels, whose microstructure contains ferrite and 10 to 20% bainite, eliminate these problems in dual phase steels and exhibit excellent characteristics. We will discuss the effects of the different second phases and the volume fraction of bainite on the mechanical properties.
It is found that as far as flash butt welding is concerned, Nb bearing ferrite+bainite steel does not show any remarkable softening or localized necking in the heat affected zone which often occurs in dual phase steel because of the tempering of martensite. The superiority of the stretch flangeability of Nb bearing ferrite+bainite steel is ascribed to the superior ductility of bainite compared to that of martensite or pearlite.